In the CF substrate used in a conventional LCD unit, a black matrix member formed from a resin or metallic film is used to shield the area of the LCD unit other than the effective opening area of each pixel against incident light. In some of a variety of applications of the CF substrate, such as for a smaller-sized LCD unit provided in a portable phone, that require relatively less higher image quality, the shield function is achieved by using a pile of adjacent two color filter layers without using the black matrix member. The absence of black matrix member saves the materials, number of process steps and thus costs of the CF substrate.
A technique of using three color filter layers in other applications of the CF substrate that require a higher image quality is known in the art to reduce the materials, number of process steps and costs without degrading the image quality (refer to Patent Publications-1 and -3). This technique also provides the merit of a higher light shield performance, which achieves a higher optical density (OD value).
Patent Publication-2 describes an example of forming a light shield member by using a half-tone mask. This configuration allows reduction of the thickness of the color filter layers and thus provides the advantage that the step difference caused by using a pile three color filter layers is reduced. Patent Publication-2 also describes a technique of patterning three color filter layers by using a common mask.
Patent Publication-4 describes a technique of using a pile of two color filter layers on the peripheral area outside the effective display area. This configuration provides the advantage that the step difference is reduced as compared to the case of using a pile of three color filter layers.
One of Patent Publications-1, 4 and 5 describes a technique wherein the lattice-shaped black matrix member is not disposed in the display area other than the peripheral area, or a single color filter pattern or a pile of adjacent two color filter patterns is disposed to shield the drain lines (signal lines). In this configuration, even if the single mask is used for shielding, the problem of reduction of optical density is solved so long as the LCD unit is of a normally-black mode and a perpendicular orientation mode of the LC layer.
Patent Publication-6 describes a configuration wherein color filter to layers covering RGB pixels are formed from four color filter layers, i.e., R-, Ye-, Cy- and B-color filter layers, a shield member for shielding the TFT is configured by a pile of three color filter layers, and the black matrix member for shielding the other area is configured by a pile of red- and blue-color filter layers. In this configuration, there is the advantage that a higher shield function is assured for the TFT area including a TFT and the vicinity thereof.
FIGS. 21A to 21D and FIG. 22 show the structure of the CF substrate described in Patent Publication-3, wherein FIGS. 21A to 21D show a top plan view of color filter layers on the CF substrate in consecutive steps of fabrication thereof, and FIG. 22 is a sectional view taken along line H-H′ in FIG. 21C. FIG. 21A shows the step of depositing a red-color filter layer 22a on a transparent substrate 40, FIG. 21B shows the step of depositing a blue-color filter layer 22b thereon, FIG. 21C shows the step of depositing a green-color filter layer 22c thereon, and FIG. 21D shows the step forming columnar spacers 31. After forming the color filter layers 22a, 22b, 22c, an overcoat film (not shown) is formed thereon depending on the type of the LCD unit, and columnar spacers 31 are formed to secure a cell gap of the LC panel which is obtained by bonding the CF substrate to a THT substrate not shown. The cell gap of the LC panel is around 3.0 to 4.0 μm. Before bonding the CF substrate to the TFT substrate, an orientation processing is performed onto the surface of both the CF substrate and TFT substrate. After the bonding or before the bonding, liquid crystal (LC) is provided in the cell gap, and a pair of polarizing films are attached onto both the outer surfaces of the LC panel.
The TFT substrate may be of an in-plane switching mode (IPS). Patent Publications-5 and 7 describe an IPS-mode LCD unit wherein drain bus lines and gate bus lines are covered by an overlying common electrode with an intervention of an interlevel dielectric film.
FIGS. 23 and 24, which correspond to FIGS. 1 and 2, respectively, in Patent Publication-5, show the IPS-mode LCD unit. FIG. 23 is a top plan view of a TFT substrate 10 in the LCD unit, whereas FIG. 24 is a sectional view of the LC panel including a LC layer 30 sandwiched between the TFT substrate 10 and a CF substrate 20. In FIG. 23, a plurality of gate lines (scan lines) 41 and a plurality of common electrode lines 42 extend in the same row direction, whereas a plurality of drain lines 43 extend in the column direction on the gate insulation film which overlies the gate lines 41 and common electrode lines 42. In the vicinity of each of the intersections of the gate lines and drain lines on the TFT substrate 10, there is provided a TFT 45 having an amorphous channel layer, as understood from FIG. 24. The TFT 45 includes a source electrode 14b connected to a comb-shaped pixel electrode 44, the other end of which is connected to a storage capacitor that is configured by the pixel electrode 44, common electrode line 42 and the interlevel dielectric film disposed therebetween. Overlying the above structure, to there is provided a common electrode 46 made of a transparent metal that overlies the drain lines 43 and gate lines 41. The common electrode 46 is connected to the underlying common electrode lines 42 via a contact hole penetrating the overcoat film and gate insulation film. In FIG. 23, sign “L” denotes the direction of rubbing treatment of the orientation film.
FIG. 25, which corresponds to FIG. 21 in Patent Publication-5, is an enlarged top plan view depicting the vicinity of the gate lines 41 in the LCD unit shown in FIG. 23. In FIG. 25, the gate lines 41 underlie the common electrode 46, which overlaps the gate lines 41 as well as the common electrode line 42. The common electrode 46 also overlies a gap between each of the source electrode and storage capacitor and the gate electrode line 41, as well as the vicinity of the electrodes that are disposed adjacent to the gate lines 41.
As shown in FIGS. 23 and 24, a black-matrix member (film) 47, which is shown as encircled by a dotted line in FIG. 23 and formed on the CF substrate 20, overlies and shields the area around the TFT 45. In this configuration, the black-matrix member 47 has roughly a minimum size that prevents incident of light onto the TFT 145. It is to be noted that the black-matrix member 47 does not overlie the gate lines 41 and drain lines 43, and has a shape of isolated pattern overlying the TFT 45.
The configuration shown in FIGS. 23, 24 and 25 is such that the electric field generated in the vicinity of the gate lines 41 is shielded by the overlying common electrode 46, and thus the orientation of LC molecules in this area of the LC layer 30 is not changed from the initial orientation, whereby leakage light from the backlight source is not generated therein. This allows the black-matrix member 47 to have the above minimum size, as shown in FIG. 23, because the CF substrate 20 need not have a light shield function.
The publications described in this text include:
Patent Publication-1 (JP-2590858B);
Patent Publication-2 (JP-1996-95021A);
Patent Publication-3 (JP-2003-14917A);
Patent Publication-4 (JP-2000-29014A);
Patent Publication-5 (JP-2004-62145A);
Patent Publication-6 (JP-61-105583A);
Patent Publication-7 (JP-2000-89240A); and
Non-patent literature-1 (I. Washizuka, IDW'97 DIGEST, 227 (1997) ALC5-4).
The technique of using a pile of three color filter layers, as described in Patent Publications-1, -3 and -5, involves the problem as discussed hereinafter.
The color reproducibility of a LCD unit, which depends on the use thereof, is around tens of percents with the upper limit thereof around 60% in the case of industrial use, such as some notebook personal computers, that does not require an NTSC ratio of 72% or above in the sRGB or an EBU standard. For example, in the combination of a color filter having an NTSC ratio of around 40% and using a photosensitive resist obtained by a typical pigment dispersion technique and a backlight including a cold cathode fluorescent lamp (CCFL), the thickness of the RGB color filter layers is around 1.0 μm. In this case, if all the shield patterns formed on the TFTs, drain lines and gate lines, i.e., all the black matrix members formed on the CF substrate are to be replaced by a pile of three color filter layers, the step difference formed between the film thickness of the effective opening area covered by a single color layer and the film thickness of the light shield member configured by the pile of three color filter layers is as large as 2.0 μm at a maximum in the vicinity of the periphery of the display area. In the case of a CF substrate of a TN (twisted nematic)-mode LCD unit, transparent electrodes of ITO (indium tin oxide) etc., which are formed on the surface of the color layer, have substantially no leveling function to solve the problem of large step difference. The overcoat film, which is typically formed on the CF substrate of an IPS- or VA (vertical alignment)-mode LCD unit, has an insufficient leveling function.
For example, assuming that the overcoat film has a thickness of 1.0 μm, only 60% to 70% of the step difference can be removed by a leveling treatment, to thereby leave a step difference of around 1.4 μm. Thus, the structure of a pile of three black-matrix members leaves a large step difference having a lattice shape in the entire display area, whereby an insufficient injection of LC or insufficient orientation caused by malfunction of the rubbing treatment, if any, will degrade the image quality of the resultant LCD unit.
The pile of three color filter layers described in Patent Publication-2 involves the problem as discussed hereinafter. The shield pattern of a pile of three color filter layers formed by the half-tone masks causes a smaller thickness of each color filter layer due to the small thickness of the half-tone masks to thereby cause a smaller optical density, and thus cannot provide a desired light shield function.
The structure of the shield section formed by a pile of two color filter layers, which are disposed adjacent to each other, among the RGB primary color filter layers, such as described in Patent Publications-3, 4 and 5 and Non-patent literature-1, involves the problem as discussed hereinafter.
If the lattice-shaped black-matrix member is not disposed in the effective display area of the LCD unit other than the peripheral area, or if a single color filter layer or two adjacent color filter layers overlapped together is provided for shielding the width of the drain lines in the to effective display area, a desired shield function is not obtained for the TFT disposed in the vicinity of the gate lines or the vicinity of the TFT. More specifically, the light shield function is degraded in the area of a pile of adjacent red- and green-color filter layers among the piles of other two color filter layers, whereby it is difficult to effectively intercept the leakage of external light or backlight by using only the color filter layers. On the other hand, the structure wherein red- and blue-color filter layers formed in the peripheral area other than the effective display area of the LCD unit, such as described in Patent Publication-4, cannot use a single common pattern for the three color filter layers, thereby raising the cost of patterning the color filter layers due to the plurality of masks being used for the three color filter layers.
The structure of using four color filter layers, i.e., R-, Ye-, Cy- and B-color filter layers, wherein a pile of three color filter layers is used for the shield section for shielding the TFT area and a pile of two color filter layers, i.e., red- and blue-color filter layers, are used in the other area, such as described in Patent Publication-6, involves the problem as discussed hereinafter.
In the above structure wherein four color filter layers, i.e., R-, Ye-, Cy- and B-color filter layers are used to obtain the RGB colors in the color filter, although one of the steps of forming the resin black-matrix member is removed, a process of forming the color filter layers itself is increased by one step, which does not substantially reduce the cost for to the process for forming the black-matrix member. Although there is a recitation in the publication that a single Cy-color filter layer used for forming the blue-color pixel, the chromaticity of Cy represented on the x-y chromaticity coordinate (CIE1931 chromaticity system) is far larger than the chromaticity of B and roughly at the median in the coordinate between G and B. Thus, it is difficult to represent the B-color by using the Cy-color.
The pattern obtained by a pile of two or three color filter layers, as described in Patent Publications-1 to -4 and Non-patent literature-1, has an acute angle or right angle in general, and involves the problem as discussed hereinafter.
If the corner of a color layer pattern formed by a pile of two color filter layers, or three color filter layers in particular, has an acute corner angle, a problem arises wherein the fiber tip of a rubbing cloth used for a rubbing treatment cannot uniformly contact the surface of the pattern, in addition to the problem of the larger step difference. The insufficient contact of the fiber tip of the rubbing cloth causes an insufficient rubbing treatment, which involves a malfunction of orientation of the LC molecules.
As discussed heretofore, the CF substrate known in the art involves the problems of a larger step difference formed on the surface of the CF substrate, and an insufficient light shield function caused by using to a pile of two color filter layers for reducing the step difference. In short, the step difference and the light shield function are tradeoffs in the known CF substrate.